Click for full size
Figure 6. This step-down-controller IC provides dual outputs with synchronous rectification.
Another possibility is the generation of a negative voltage from
a positive one, using an inverting DC-DC
controller, as shown in
Figure 7. This IC and a few external components (inductor, power MOSFET, and
output capacitor) provide the simplest and easiest way to produce the -5V @ 2A required for this
system. An evaluation kit
from Maxim, incorporating all the parts just mentioned, simplifies the board
layout and accelerates the design process.
Figure 7. These switch-mode ICs convert positive input voltages to a regulated negative output.
In most cases, the central power supply is a custom design that is mounted in a location that provides
easy access and convenient thermal management. The specification is written by systems or design
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engineers, and it is submitted either to the company's power-supply department or to one of the many
other companies that specialize in the design and manufacture of power supplies. Either way, the
eventual result is delivery of a thoroughly tested and qualified "black box."
High voltage from the battery affects only the input section of the supply. For safety reasons and to
comply with specifications such as EN60950, UL950, and so forth, all secondary outputs are isolated
from the battery. Compliance with these standard safety specs also ensures that the centralized supply
can distribute its regulated voltages throughout the system, and this is without any concern for clearance
distances to other parts of the equipment.
To achieve redundancy, you can easily parallel two power supplies using series diodes to create OR
connections between corresponding outputs. This architecture doubles the cost and the size of the
power supply, yet it is often used with switching post-regulation in small-to-medium telecom systems.
For more complex systems, such as the example mentioned earlier, this approach presents two
problems: First, a costly harness of cables and connectors is required to carry high load currents to all
boards in the system. Second, voltage regulation among the boards is a problem; remote sensing can
guarantee regulation at one board, but it doesn't necessarily provide enough tolerance at the other
boards to ensure proper operation for the various ICs mounted on them. This last point often excludes
consideration of a centralized power supply. Even for simple systems, the trend toward lower supply
voltages and the need for tighter regulation make the centralized approach increasingly difficult to
implement.
Distributed and Isolated Power Supply
In this approach, the battery voltage (-48V) is provided to all boards in the system (
Figure 8) and every
board includes one or more power supplies suited to the requirements of that board. In a sense, the
centralized supply has been split into various smaller DC-DC converters, each independent of the others.
For this purpose, today's market offers a wide range of single- and multiple-output DC-DC converters
with capacities from a few watts to more than 600W. As an example, either of two circuits can meet the
"interface-board" power requirements:
1. Three DC-DC converters
2. One DC-DC converter and three switching post-regulators
The first solution is easy; you can purchase three "off-the-shelf" modules and mount them on the board
according to the manufacturers' specifications for EMI filtering, short-circuit protection, thermal
management, etc. The drawback is cost (per watt), because each module requires its
own isolation
transformer, feedback loop, and other components. Cost is minimized in the second circuit, because
isolation and the main output voltage are provided by the one DC-DC converter and simple step-down
converters supply the other regulated outputs. Power requirements are generally in the 10W to 30W
range, so a flyback or forward-converter topology can serve for the first stage.
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